How to Write while and do while Loops in Java

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In Brief...

The while loop in Java executes one or more statements after testing the loop continuation condition at the start of each iteration. The do while loop, however, tests the loop continuation condition after the first iteration has completed. Therefore, the do while loop guarantees one execution of the loop logic whereas the while does not. To understand how to code each of these loops, follow these eight steps.

Instructions

  1. Open your text editor and type in the following Java statements.
    Java Source While and Do While
    Notice that each loop will execute five times because the loop continuation condition is i < 5 and the variable i is initialized to 0 but pre-incremented to 1 on first reference.

  2. Save your file as WritewhileAnddowhileLoops.java.

  3. Open a command prompt and navigate to the directory containing your Java program. Then type in the command to compile the source and hit Enter.
    Compile Source for While Do While Program

  4. You are ready to test your program. Type in the command to run the Java runtime launcher and then hit Enter.
    Run the While Do While Program
    Notice each loop executes five times.

  5. To understand the difference between the two types of loops, modify WritewhileAnddowhileLoops.java in your text editor so that the value of i is 5.
    Java Source for Modified While Do While Program
    The while loop statement will not execute at all now. However, one iteration of the do while will execute.

  6. Save your changes.

  7. Return to your command prompt and compile the modified source.
    Compile Source for Modified While Do While Program

  8. Test your modified program. Type in the command to run the Java runtime launcher and then hit Enter. Notice that the while statement did not execute, whereas one iteration of the do while did execute.
    Run the Modified While Do While Program

Author: Stephen Withrow

Stephen has over 30 years' experience in training, development, and consulting in a variety of technology areas including Java, C, C++, XML, JavaScript, AJAX, Tomcat, JBoss, Oracle, and DB2. His background includes design and implementation of business solutions on client/server, Web, and enterprise platforms. Stephen is a published writer in both technical and non-technical endeavors. Stephen received an undergraduate degree in Computer Science and Physics from Florida State University.

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